426 Lang . — Studies in the Development and 
anticlinal walls. The sorus appears as a slight elevation, 
which is at first flat-topped, but at an early stage becomes 
depressed centrally by the more rapid growth of the 
surrounding cells. The origin of the sorus depends, partly on 
the increase in size of the cells of the hypodermal layer, and 
partly on division and growth of the cells underlying this. 
Sometimes the latter owe their origin to earlier divisions of 
the hypodermal layer, in which case a section of the sorus 
shows vertical rows of cells ; in other sections this is not so 
obvious. The merismatic appearance of the tissue extends 
more deeply beneath a developing sorus than elsewhere, and 
in a slightly later stage is seen to be continuous with the 
procambial strand of the bundle, which terminates beneath 
each sorus. In radial sections of such young sori certain 
hypodermal cells of large size and with prominent nuclei can 
be recognized. They occupy a definite position, about mid- 
way between the depressed centre and the margin. Usually 
two such cells are visible in a vertical section passing 
through one of the young sporangia (Fig. 5), but com- 
parison with the adjoining sections of the series and with 
transverse sections of sori of similar age shows that a group 
of four cells is present, only two of which can appear in 
a vertical section. From this group of cells the sporogenous 
tissue is derived. The frequency with which anticlinal 
divisions take place in the hypodermal layer renders it a 
matter of great difficulty to determine whether this group is 
derived from a single cell, as Goebel 1 suggested in the case of 
Zamia , or not. In certain cases I am of opinion, from a care- 
ful study of the arrangement of the cell-walls, that this is not 
the case, but in a few examples it appeared to be a possible 
interpretation. No single cell could, however, be distinguished 
as occupying the position of the cell-group in younger sori. 
Each of the four cells undergoes division by a periclinal 
wall into a flat outer cell and an inner one of slightly greater 
size (Fig. 6). From the four inner cells thus produced the 
sporogenous tissue is derived. Their origin is similar to that 
1 Goebel, Vergleich. Entwicklungsgesch. d. Pflanzenorgane, p. 395. 
